Flow regimes are normally correlated by using the dimensionless parameter Reynolds' Number.
As the term Reynolds' Number will be used throughout this specification, a brief explanation of the term Reynolds' Number follows. A Reynolds' Number is a dimensionless parameter which represents the ratio of viscous to dynamic forces involved in fluid flow and is used to relate key variables in the design of diverse fluid flow systems. The Reynolds' Number is the ratio of dynamic to viscous forces within a flowing fluid and it is of paramount importance in drag phenomena and vorticity. Specifically, the Reynolds' Number that is associated with a particular fluid flow system accounts for the effect of fluid viscosity, velocity, and density on the drag characteristics of fluid flow within the flow path of a flowing fluid through a device. A Reynolds' Number is obtained by multiplying the density of the flowing fluid by its velocity by a shape parameter (such as diameter or length). This product is then divided by the kinematic viscosity of the fluid to obtain the dimensionless Reynolds' Number. In very low gas or liquid flows, in the Hagen-Poiseuille range, the Reynolds' Number for the fluid flow system may be in the range 10.sup.1 to 10.sup.2. For very high flows in a critical turbulent range, the Reynolds' Number may be in the range 10.sup.6 or 10.sup.7, and, in between, in subsonic flow ranges.
The current invention uniquely not only operates over broad ranges, but also works equally well with liquids and gases and fluidized powders. It can also be built in a bi-directional version to measure mass flows in both directions.
The ability to traverse broad flow Reynolds' Number ranges from the viscous to the transonic range is unique among mass flowmeters. The ability to operate well at very low flows is also unique.
The invention also features modular design ability through algorithmic interrelationships allowing for easy design to meet a variety of fluids, flow ranges and state variables.
The accurate and precise measurement of the volume and/or mass of fluid flow (as distinguished from the rate of fluid flow) or the accurate and precise measurement of the quantity of a continuously moving fluid such a gas, vapor, liquid, or flowable solid through a channel or conduit with a broad range, low cost, easily usable device is a problem that has challenged scientists, engineers, and designers for many years.
Many commonly available flowmeters measure only the flow rate of the flowing fluid in terms of a standard quantity per unit time as opposed to measuring actual mass flow. Accordingly, mass flow measurement has become increasingly necessary for medical applications, and also due to costs of materials, fuel economy and process control.
There are only three types of full flow, mass measuring flowmeters presently in use. Such full flow, mass measuring flowmeters include those based on Coriolis principles, those based on magnetic principles or those employing ultrasonics.
Full flow, mass measuring flowmeters based on Coriolis principles, magnetic principles or ultrasonics generally suffer from several common drawbacks. Such drawbacks include:
(a) The inability to easily and accurately measure the mass flow of all three states of matter . . . gases, liquids and flowable solids such as powders. PA1 (b) The high cost of such flowmeters caused by the complex highly accurate physical structure and the complex algorithms and the implementing electronics needed to obtain accurate mass flow measurements. PA1 (c) The inability to measure flows over a broad band range of Reynolds' Numbers, necessitating a large number of flow range limited designs. PA1 (d) Installation prerequisites required, such as flow straighteners, or pre-conditioning devices. PA1 (e) Finally, another drawback of such fluid flow rate meters is usually the creation of a high pressure drop within the flowing fluid and the instability of these fluid flow rate meters over large dynamic ranges. Many of them only work on gases or liquids only. PA1 a) is operable over a wide range of Reynolds' Numbers; PA1 b) can be used with compressible gases, liquids and fluidized powders; PA1 c) is not unpredictably affected by fluid viscosities, which with additional equipment it can also measure; PA1 d) can be used with multi-component fluids; PA1 e) can be used without prior fluid conditioning or flow straightening devices; PA1 f) can be used with fluidized solids or non-Newtonian fluids; PA1 g) can be bi-directional; and PA1 h) produces a strong, precise, accurate, clearly recognizable signal, which offers maximum accuracy and precision capability in conjunction with solid state transducers and application specific integrated circuits (ASIC's).
Excluded from the foregoing listing of full flow, mass measuring flowmeters are thermal mass flowmeters as they are not truly full flow, mass measuring flowmeters, but, rather, such thermal mass flowmeters use bypass capillary thermally heated sampling ducts in parallel with the flow stream. Such capillary thermally heated sampling ducts in thermal mass flowmeters clog easily, require periodic cleaning and create frequent recalibration problems for their users. Additionally, such thermal mass flowmeters only provide an analog of the total measurement of fluid flow.
Thermal mass flowmeters do not generally work with liquids or fluidized powder as they are essentially "by-pass" meters. Thermal mass flowmeters also impose fluid conditioning requirements, long time delays and low rangeability as well as other problems for the user.
Also, not included in the foregoing list of flowmeters, are those devices which simply measure the rate of fluid flow. Such devices include venturis, various types of precision flow nozzles, flapper vane devices, and orifice plates. In addition to not measuring mass flow, these flow rate measuring devices all have extremely significant operating range (Reynolds' Number span) limitations. Specifically, the dynamic range or the ratio of the maximum signal level ability to the experienced noise level of these flow rate measuring devices is generally limited to low dynamic ranges such as 10:1 to 20:1. In unusually expensive instruments and application limited devices somewhat higher ranges can sometimes be achieved, but these devices remain limited.
Still further excluded from the identified group of full flow, mass flowmeters are devices called vortex shedders. Vortex shedders measure the frequency at which unstable vortices are generated by a large flow obstruction in the path of the flow, and generate unstable vortex trails. Once again, these devices measure only flow rate and typically are stable over limited dynamic ranges. Vortex shedders use an unstable pulsating vortex separation process that must be sensed by expensive transducers such as ultrasonic devices in order to obtain an accurate measurement of fluid flow rate. Vortex shedding occurs due to boundary layer separation. The mass flowmeter of the present invention described herein works with liquids, gases, fluidized solids, without the limitations of vortex shedder mass flowmeters.
One of the perceptions that has hampered the creation of a full flow, mass measuring flowmeter is the perception that fluid flow is essentially a two dimensional phenomenon. This also results from the commonly used one dimensional Bernoulli flow model used to design most of these devices. This perception has resulted in not fully appreciating the impact of the rotation or vortex action on the flowing fluid. In many devices, such as venturis, random rotation defeats the various flow rate processes. In reality, fluid flow is a complex three-dimensional phenomenon. Examples of the third dimension in fluid flow are readily observed as wing-tip vortices which appear during flight operations at the trailing edges of the wings of a fixed wing airplane or bird in flight. These processes are integral to flight itself, tornadoes, hurricanes and similar phenomenon. These commonly observed vortices exhibit a natural three dimensional rotational phenomenon but are inherently unstable. Flow in nozzles and venturis is three dimensional, but is treated conventionally as one or two dimensional. Uncontrolled rotation and boundary layer separation often destroys the operation of such flow devices.
Prior art attempts to construct three dimensional full flow mass measuring flowmeters have been beset by several problems. These problems have limited the applicability of prior art flow measurement devices to a limited dynamic range dynamic at medium to high flow measurements (high Reynolds' Numbers). In most of these measurements the flow process has been completely terminated by boundary layer separation or random rotation beyond the specified limited dynamic range.
Many of these other prior art full fluid flow devices produce very small unamplified signals. The devices using this invention feature substantial fluidic signal amplification (prior to electronic sensing), because the signals produced are many times greater than dynamic pressure ("dynamic heads"). The devices using this invention amplify the signals by generating high level fluidic signals. Further, in other prior art fluid flow devices, the signal sensing probes are often in the unstable boundary layer region. Thus, the signal is oftentimes washed out due to instability of normal fluid flow device boundary layers. This does not occur in the devices using the instant invention.
The limitations of prior art full flow, rate measuring flowmeters are due primarily to a failure to stabilize the boundary layer between the flowing fluid and the inner wall of the body or conduit which forms the flow path for the fluid and random rotation. It is well known that boundary layers are formed along the wall of the fluid conduit by the viscous friction between the flowing fluid and the inner wall of the conduit. This viscous friction provides a resistance to fluid flow and the resultant boundary layer state actually determines whether or not fluid flow continues or terminates. If the boundary layer becomes turbulent and separates from the inner wall of the fluid conduit, the fluid flow process stops. If the fluid flow process remains regular absent random rotation, then the fluid flow continues through the fluid conduit and the boundary layer remains attached to the inner wall of the fluid conduit and the process can be completed. However, stabilizing a boundary layer along the inner wall of fluid conduit is not an easy task. In most fluid flow systems, either random rotation of the flowing fluid or turbulence or the impact of fluid velocity and viscosity and resultant friction within the flowing fluid, as well as the shape of the flow conduit, determines when the boundary layer becomes unstable and separated thus ending the stable flow process. This instability or separation of the boundary layer limits the range of venturis and nozzles, as used in flow rate measurement.
In the measurement of low flows, viscous phenomena in the low flow range, which take place in standard flow devices or fluid conduits and the viscous friction forces between the flowing fluid and the inner wall of the fluid conduit, predominate the flow process. Such viscous friction forces play a dominant role in creating the potential for unstable flow random rotation or vortex shedding. This is why there are very few reliable mass or rate measuring flowmeters which can accurately measure flows in the low Reynolds' Number flow ranges. Certainly there are fewer low flow mass flow devices, and when they do work, their range ability is severely limited.
In mid-range fluid flows there is typically a region of relative quiescence or relative stability. However, even in this range the random rotation and boundary layer separation problems nonetheless continue to persist.
At high flow rates which extend into the transonic range, the boundary layers become turbulent and the flow process is interdicted because of the separation of the boundary layer from the inner wall of the fluid conduit. When such separation of the boundary layer from the inner wall of the fluid conduit occurs, fluid flow terminates. These flow-inhibiting boundary layer effects are why none of the prior art fluid flow mass flow measuring devices such as nozzles, venturis, and vortex shedders, are able to operate over a wide range of flows or Reynolds' Numbers, as can the inventions described herein.
Still another problem which affects fluid flow in a flow conduit is the oscillatory nature of fluid flow. Such oscillatory flow occurs as fluid flows approach the speed of sound or the transonic or sonic choke point and at very low flows. This is due to the formation of shockwaves within the flowing fluid, and the effective role of viscous friction.
Yet another problem which affects the stability of fluid flow is the changing nature of the controlling orifice conditions and associated flow coefficients representing a variation of fluid flow rate and state variable change in these nozzles, venturis, and similar devices. For example, in venturi-like devices and nozzles, where pressure sensing taps are located in the wall of the nozzle, both the size and shape of the boundary layer and the internal rotation of the flowing fluid within the venturi seriously affects the accuracy of the flow readings and the stability of the signals in these pressure sensing probes. Accordingly, venturi-like devices and flow nozzles are particularly subject to flow instability problems whenever they are used. Thus, venturi-like devices and nozzles have been recommended for use with flows having a Reynolds' Number greater than 10.sup.5, the high to moderate flow range. Other similar restrictions have been placed on other prior art fluid flow devices.
Venturis, nozzles, and orifice plates all encounter boundary layer separation problems, flow oscillation problems and random rotation problems within the flowing fluid. The problem with placing fluid pressure sensing taps near the wall of a fluid conduit is that the sensed pressure reading is affected by the unstable boundary layer within the fluid flow conduit. All these problems are overcome in the instant invention by the multiple vortex formations caused by the multiple rod system and the especially designed contours of the mass flowmeter of the instant invention.
To overcome some of the problems found in prior art flow measurement devices such as venturis, nozzles and orifices plates which are intended to be full flow meters, but are at best restricted to flow rate measurement only, various full flow mass measuring devices which utilize magnetism, Doppler sonic effects, and the minimal effects of the spin of the earth's Coriolis effect as it affects local rotational spin in a fixed body have been developed. In the Coriolis effect full flow mass measuring devices, such local rotational spins are measured by highly sensitive movement strain sensors. These more advanced and complex mass flow fluid flow measuring devices do not present the drawbacks of venturis, nozzles, and orifice plates, but rather they present significant disadvantages that are addressed above and these mass flow measuring devices are dominated by cost, limited rangeability, and complexities of installation, thus their utility is limited.
In addition to the limited range of Reynolds' Number flows which prior art flowmeters are capable of measuring, there is yet another problem. This problem is the difficulty of obtaining accurate mass measurement of gas flows because of the need to deal with the inherent compressibility of gases. Such compressibility particularly distorts measurements of mass flow due to pressure and/or temperature and therefore the density and viscosity changes which are encountered in the compressible flow measurement process. This is even a more serious problem in rate flow measurement devices as opposed to mass flow measurement devices.
Viscosity variations also dramatically affect the operation and/or accuracy of most flowmeters. Similarly, many prior art flowmeters cannot effectively measure multicomponent fluid flows such as liquids and gases together in one fluid conduit, or deal with non-Newtonian fluids. The invention described herein measures these multicomponent fluids as a combined fluid.
Because of the sensitivity of many flowmeters to turbulence, it is often necessary to insert flowmeters downstream from a flow straightener or flow conditioner to remove turbulence and rotation within the flowing fluid. In still other applications upstream turbulence is reduced by positioning flowmeters only after a long run of straight pipe.
There is also a need in the art to measure mass flows of fluidized solids such as powders or non-Newtonian fluids such as blood, and other conglomerate fluids.
In a number of applications there is a need for a bi-directional capability, specifically there is a need to have a single flowmeter measure mass fluid flow in both directions within a fluid conduit.
Finally, a need remains for a flowmeter which produces a strong, widely separated, accurate and precise signal. Such signal should be easily measured by either shielded or unshielded solid state pressure measurement transducers.
Summarizing this invention: It satisfies a need in the art for a full flow, mass measuring flowmeter which: